38o MURIDiE 



(B) Palate, complicated, with lateral pits; 

 cheek-teeth rootless : — 



(a) Size larger; "rats," with condylo-basan . „ rjy--^ r^ 



length rarely less than 39 mm. ; m^ with V -/ih'h' * 



three closed triangles . J "'"'^ ' *"^' 



(b) Size smaller; "mice," with condylo-1 



basal length rarely reaching 30 mm.; wzj \MICR0TUS. 

 with five closed triangles . .J 



(a') Ttfi with four dentine-spaces : — 



(a) /«j with anterior loop nearly bi- 



laterally indented : — 



[M. sandayensis westra 



(a') Cranium depressed . A (Westray Island, 



[ Orkneys). 



(b') Cranium not depressed .{^- o^nty Isknir* 



(b) OTj with outer infold of anterior! M. sandayensis sanday- 



loop much less developed than \ ensis (Sanday Is- 

 inner infold . . .J land, Orkneys). 



(b') nfi with a small 3rd inner salient angle, 

 making five dentine-spaces : — 



(a) wi with three inner salient angles ; 



bullae small : — 

 (a') Size smaller (condylo-basal length"!^ , . 



not exceeding 26 mm.) ./ 



(b') Size larger (condylo-basal length"! J/, a. neglectus (Scottish 



reaching 28-29 nim.) ./ Highlands). 



(b) ni^ frequently with a small 4th inner 



salient angle ; bullae large : — 

 (a') Temporal ridges anteriorly\il/. a. macgillivraii 



parallel in adults . ./ (Islay). 



(b') Temporal ridges anteriorly con-\^ ^ ,;,,«/ (Hebrides). 



vergmg in adults . . J ^ 



2. Skull rounded; cheek-teeth with tubercular 

 crowns : — 



(A) Rostrum short (distance from orbital con-"l 



striction to anterior edge of nasals abontYMICJiOMYS minutus. 

 J total length of skull) . . .J 



(B) Rostrum normal (distance from orbital 

 constriction to anterior edge of nasals 

 about J total length of skull) : — 



(a) Upper incisors notched; m^ much\^Ty-<j 

 longer than w^ and w' together ./ 



^'^ p2d7rftiLly'°"^ /^-P^y}^....^.(StKilda). 



^'^ StSSf"."^'^""''^'"":}^ — ^- 



The sub-species of Arvicola amphibius are omitted, as they do not possess diagnostic 

 cranial or dental characters. 



